Radio station with circularly polarised antennas

ABSTRACT

A radio station including two antennas combined with respective first and second hybrid transmission polarization couplers is disclosed. Each antenna is arranged to generate two orthogonal electric field components in response to two respective quadrature radio signals from the corresponding polarization coupler. The station further includes at least one hybrid distribution coupler with a first output connected to a first input of the first polarization coupler and a second output connected to a first input of the second polarization coupler and at least one radio signal source delivering a radio signal to a first input of the distribution coupler.

BACKGROUND OF THE INVENTION

This application is a 371 of PCT/FR97/00524 May 25, 1997.

The present invention concerns a radio station, which may be usedparticularly as a base station in cellular radio systems.

Traditionally, radio-communications systems with mobile radio stationsuse base stations equipped with vertically and linearly polarisedantennas. When it is required to couple more than one radio signalsource to an antenna, devices of the hybrid coupler type with only oneoutput connected to the antenna are used. In this case, the other hybridcoupler output must be connected to a load resistor to match theimpedance. This load resistor dissipates half the radio power which isnot radiated usefully and causes an undesirable rise in temperature.

One drawback of the use of linear polarisation in radio-communicationswith mobile radio stations is that the quality of communication dependson the orientation of the mobile radio station. For example,measurements have shown that a vehicle antenna of the coaxial typeinclined at 45° may cause 80% signal loss for a vertical linearpolarisation transmission.

Moreover, it is known that diversity treatments enable the performanceof radio-communication systems to be improved. Base stations of cellularsystems normally use spatial type diversity in reception, by means oftwo spatially separate vertically polarised antennas. The use ofpolarisation diversity rather than spatial diversity has also beensuggested. For this two antennas located in the same place are used, onesensitive to vertical polarisation and the other sensitive to horizontalpolarisation.

One object of the present invention is to improve the possibilitiesoffered for transmission by a radio communication base station.

SUMMARY OF THE INVENTION

The invention thus proposes a radio station, including two antennascombined respectively with first and second hybrid transmissionpolarisation couplers, each antenna being arranged to generate twoorthogonal electric field components in response to two respectivequadrature radio signals from the corresponding polarisation coupler.The station further includes at least one hybrid distribution couplerwith a first output connected to a first input of the first polarisationcoupler and a second output connected to a first input of the secondpolarisation coupler, and at least one radio signal source delivering aradio signal to a first input of the distribution coupler.

Thus, each antenna transmits a portion of the radio signal from thecircularly polarised source. As a result, the quality of the receptionby the mobile radio station no longer depends on the orientation of itsantenna in relation to a linear polarisation direction.

In a first version of the invention, the hybrid couplers are connectedto each other and to the antennas in such a way that the radio signalfrom the source is transmitted by the two antennas in the form of tworespective radio waves circularly polarised in the same direction. Anappropriate relative positioning of the two antennas, and an appropriatechoice of length of the coaxial cables connecting the couplers to eachother then enables a gain in transmission directivity (up to about 3 dB)to be obtained. Such a gain in directivity means that the base stationis highly suitable for the microcellular case, especially when radiopenetration inside buildings is required.

In another version of the invention, the hybrid couplers are connectedto each other and to the antennas in such a way that the radio signalfrom the source is transmitted by the two antennas in the form of tworespective radio waves circularly polarized in opposite directions. Thisgives a transmission polarisation diversity which enables the effects offading to be counteracted. This version is very appropriate in caseswhere the propagation medium creates relatively little diversity, i.e.when the waves transmitted sustain relatively few reflections(propagation in a rural, desert, maritime environment etc). The gain indiversity thus obtained can run from 3 to 10 dB.

It is noted that it is very easy to switch from one of the versionsmentioned above to the other, simply by modifying the connections of thecoaxial cables connected to the couplers.

The same type of equipment can thus be used to meet the diverserequirements of the network operator.

The advantages disclosed above can easily be obtained for several radiosignal sources. The two coupling stages have the advantage, when severalradio sources are connected, of enabling all the available power (exceptfor losses in duplexers) to be radiated usefully, which avoids uselessheat being dissipated in the rack.

The use of the two antennas of the invention further enablesadvantageous arrangements in the reception part of the radio station.These arrangements are combined to advantage with those which have justbeen mentioned for the transmission part, but they would be applicableindependently. In accordance with one of these arrangements, the radiostation includes at least one receiver ensuring a diversity treatment oftwo input radio signals, one of the said input radio signals beingobtained from an electric field component picked up by one of the twoantennas according to a first direction, and the other input radiosignal being obtained from an electric field component picked up by theother antenna according to a second direction orthogonal to the saidfirst direction.

The receiver then combines the advantages of both spatial diversity andpolarisation diversity in counteracting fading. It is possible toinstall several receivers in this way.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a radio station in accordance with the inventionwith one transceiver unit;

FIG. 2 is a diagrammatic view of a hybrid coupler which can be used in astation in accordance with the invention;

FIG. 3 is a diagram of a variant of a version of the station in FIG. 1;

FIGS. 4 to 6 are diagrams of radio stations in accordance with theinvention with two transceiver units; and

FIGS. 7 and 8 are diagrams of radio stations in accordance with theinvention with four transceiver units.

DESCRIPTION OF PREFERRED EMBODIMENTS

The radio stations shown in FIGS. 1 and 3 to 8 include two antennas 10,12 each constituted by two co-located crossed dipoles. For each antenna,the two dipoles are orthogonal, one being intended to be placedvertically, the other horizontally.

Each antenna 10, 12 is connected to a respective hybrid transmissionpolarisation coupler 14 ₃, 14 ₄. Each of these couplers 14 ₃, 14 ₄ hastwo outputs, one C3, C4 supplying power to the horizontal dipole of itscorresponding antenna 10, 12, and the other D3, D4 supplying power tothe vertical dipole of its corresponding antenna 10, 12.

Each polarisation coupler 14 ₃, 14 ₄ is selected so that it can producetwo quadrature radio signals at its two outputs C3 and D3, C4 and D4.They may particularly be hybrid couplers of the 0°/90° type such as theone shown as a diagram in FIG. 2. Such a coupler includes a dielectricsubstrate, provided with a copper earth plane, on which is laid a copperconductor pattern like that shown in FIG. 2. This pattern is composed oftwo parallel segments Ai-Ci, Bi-Di spaced by λ′/4, where λ′ denotes thewave length of the radio signals taking into account the relativepermittivity of the substrate, and of two other segments, also parallelto each other and spaced by λ′/4, extending perpendicularly between thesegments Ai-Ci and Bi-Di. The adjacent ends Ai, Bi of the two firstsegments constitute the two coupler inputs, whereas the two oppositeends of these segments Ci, Di constitute the two outputs of the coupler14 _(i) (i=3, 4). With such a coupler, called a “branchline” coupler,provided that the impedances of the four ports are adapted (typically to50 Ω), the radio signal reaching the input A_(i) is divided into twohalf power portions, one in phase from the output Ci, and the otherout-of-phase by −90° from the output Di and, symmetrically, the radiosignal reaching the input Bi is divided into two half power portions,one in phase from the output Di, and the other out-of-phase by −90° fromthe other output Ci.

The components delivered by the outputs Ci and Di of the coupler 14 _(i)are thus always in quadrature in relation to each other, such that whenthey supply power to the two orthogonal dipoles of the correspondingantenna, the two orthogonal electric field components generated by thesedipoles lead to the transmission of a circularly polarised radio wave.The circular polarisation direction is different for the radio signalreaching the input Ai of the coupler and for the radio signal reachingthe input Bi of the coupler. It will be noted for example that thesignal reaching the input Ai is transmitted with leftwards circularpolarisation (LCP), and that the radio signal reaching the input Bi ofthe coupler is transmitted with rightwards circular polarisation (RCP).

In the version example shown in FIG. 1, the inputs A3 and A4 of thepolarisation couplers 14 ₃, 14 ₄ are connected by respective coaxialcables to two outputs C1, D1 of a hybrid distribution coupler 14 ₁. Thisdistribution coupler 14 ₁ complies for example with the hybrid describedby reference to FIG. 2 (i=1). Its input A1 is connected to a source orradio signal transmitter T1 which is part of a transceiver unit TR1. Theother input B1 of the distribution coupler 14 ₁ is connected to theearth by means of an impedance matching resistor 16. The same is true ofinputs B3, B4 of polarisation couplers 14 ₃, 14 ₄.

With the assembly in FIG. 1, the radio signal from the source T1 istransmitted with LCP by the two antennas 10, 12.

The fact that the two antennas 10, 12 transmit the same radio signal inaccordance with the same polarisation can be used to obtain a gain indirectivity for this signal. This is obtained by an appropriate choiceof distance d between the two antennas 10, 12 and lengths L, L+ΔL ofcoaxial cable connecting the outputs C1, D1 of the distribution coupler14 ₁, to the inputs A3, A4 of the polarisation couplers 14 ₃, 14 ₄.

It is known that, when two identical radio signals are to a degreeout-of-phase when they reach the two antennas transmitting them inaccordance with an identical polarisation mode, the directivity of thetransmission system varies with the distance d between the two antennas.A distance d may particularly be selected which leads to a considerablegain in directivity, of about 3 dB for example. In the case of zerophase shift, maximum gain in directivity (2.95 dB) is obtained with theselection d=0.92λ, where λ denotes the wave length in the air of theradio waves. This zero phase shift condition is fulfilled whenΔL=(n−ΔΦ/2π)λ″. In the expression above, ΔL denotes the difference inlength between the coaxial cable connecting the output D1 of the coupler14 ₁ to the input A4 of the coupler 14 ₄ and the coaxial cableconnecting the output C1 of the coupler 14 ₁ to the input A3 of thecoupler 14 ₃, n denotes any integer, λ″ denotes the wave length of theradio signals in the coaxial cables, and ΔΦ denotes the phase differencebetween the portion of the radio signal present at the output D1 of thedistribution coupler and the portion of this same radio signal presentat the output C1 of the distribution coupler (ΔΦ=−π/2 when thedistribution coupler 14 ₁ is of the type shown in FIG. 2).

The gain in directivity makes the radio station highly suitable forapplications in sectored base stations or in microcellular network basestations, particularly when there is a requirement for radio wavepenetration into buildings.

The receiver R1 of the transceiver unit TR1 is designed to ensurediversity treatment of two input radio signals, as is usual in the fieldof cellular radio systems. The presence of the two antennas 10, 12 inthe radio station enables the advantages of spatial diversity andpolarisation diversity to be combined in the two receiver R1 inputsignals. One of these input signals is the horizontal electric fieldcomponent picked up by the horizontal dipole of the antenna 10, whereasthe other input radio signal is the vertical electric field component inanother location picked up by the vertical dipole of the other antenna12. Two duplexers 20 _(H), 22 _(V) v are connected to the horizontaldipole of the antenna 10 and to the vertical dipole of the antenna 12respectively so as to separate the transmission and reception paths.

The version shown in FIG. 3 differs from that shown in FIG. 1 in thatthe output D1 of the distribution coupler 14, is connected to the inputB4 and not to the input A4 of the polarisation coupler 14 ₄, the inputA4 being then connected to an impedance matching resistor 16. In thiscase, the radio signal from the source T1 is transmitted with LCP by theantenna 10 and with RCP by antenna 12. The base station then givesspatial and polarisation diversity to the transmission, with the resultthat it is highly suitable for propagation mediums producing fewreflections (rural, desert, maritime environments, etc.).

It is pointed out that the installer of the station may opt for a gainin directivity or a gain in diversity simply by connecting up thecoaxial cable connected to the output D1 of the distribution coupler 14₁ in a different way. It is thus clear that the abundant potential ofthe same equipment can be realised with only basic adjustments.

In the versions shown in FIGS. 4 to 5, the radio station comprises asecond transceiver unit TR2, TR3, with a radio signal source T2, T3 anda diversity receiver R2, R3. The previously disclosed advantages may befully obtained for the two transceiver units TR1, TR2.

In the example shown in FIG. 4, a second distribution coupler 14 ₂ isprovided, for example of the type described by reference to FIG. 2(i=2). The distribution coupler 14 ₂ has its input A2 connected to theoutput of the source T2, its input B2 connected to an impedance matchingresistor 16, its output C2 connected to the input B3 of the polarisationcoupler 14 ₃, and its output D2 connected to the input B4 of thepolarisation coupler 14 ₄. Thus, the radio signal from the source T2 istransmitted with RCP by the two antennas 10, 12, whereas the radiosignal from the source T1 is transmitted with LCP by the two antennas10, 12. This enables the advantage of a gain in directivity to beobtained for the two radio signal sources. In the case shown in FIG. 4,the radio signal from the source T2 is out-of-phase by −90° at theoutput D2 of the coupler 14 ₂, and has zero phase shift at the output C2(i.e. a ΔΦ′ phase shift of +90° compared with the output D2). Thedistance d between the two antennas being 0.92λ, the difference inlength ΔL′ between the coaxial cable connecting the output C2 of thecoupler 14 ₂ to the input B3 of the coupler 14 ₃ and the coaxial cableconnecting the output D2 of the coupler 14 ₂ to the input B4 of thecoupler 14 ₄ is of the form (n′−ΔΦ′/2π)λ′, where n″ denotes any integer,to obtain a gain in directivity of 2.95 dB.

For the reception part, two other duplexers 20 _(V) and 22 _(H) areconnected to the vertical dipole of the antenna 10 and to the horizontaldipole of the antenna 12 respectively so as to separate the transmissionand reception paths. These two duplexers provide the receiver R2 of theunit TR2 with its two spatially and polarisation diverse input radiosignals.

The version example shown in FIG. 5 differs from that in FIG. 4 in thatthe output D1 of the distribution coupler 14 ₁, is connected to theinput B4 of the polarisation coupler 14 ₄, whereas the output B2 of thedistribution coupler 14 ₂ is connected to the input A4 of thepolarisation coupler 14 ₄. This version thus gives the gain in diversityfor the two radio signal sources T1 (LCP on antenna 10, RCP on antenna12) and T2 (RCP on antenna 10, LCP on antenna 12).

The version example shown in FIG. 6 provides comparable advantages tothat in FIG. 5. In this example, there is no second distribution coupler14 ₂, the inputs B3 and A4 of couplers 14 ₃ and 14 ₄ being connected toimpedance matching resistors 16. The source T3 of the second transceiverunit TR3 is connected to the input B1 of the distribution coupler 14 ₁,with the result that the radio signal it delivers is transmitted withLCP by antenna 10 and with RCP by antenna 12. Two division couplers 14₅, 14 ₆, which may be of the type shown in FIG. 2 (i=5, 6), are providedfor the reception part. The division coupler 14 ₅ has its input A5connected to the duplexer 22 _(V), and its input B5 connected to animpedance matching resistor 16. Its output C5 supplies the first inputsignal to the receiver R1 and its output D5 supplies the first inputsignal to the receiver R3. The division coupler 14 ₆ has its input A6connected to the duplexer 20 _(H), and its input B6 connected to animpedance matching resistor 16. Its output C6 supplies the second inputsignal from the receiver R1, and its output D6 supplies the second inputsignal from the receiver R3. Diversity in reception is thus obtained foreach of the two receivers. Compared with the version example in FIG. 5,the one in FIG. 6 requires one additional hybrid coupler, and two fewerduplexers.

In the version examples shown in FIGS. 7 and 8, the radio stationcomprises four transceiver units TR1, TR2, TR3, TR4, two distributioncouplers 14 ₁, 14 ₂, and four division couplers 14 ₅, 14 ₆, 14 ₇, 14 ₈.

In the example in FIG. 7, the distribution couplers 14 ₁, 14 ₂ areconnected to the polarisation couplers 14 ₃, 14 ₄ in the same way as inthe example in FIG. 4. The input B1 of the distribution coupler 14 ₁ isconnected to the radio signal source T3, whereas the input B2 of thedistribution coupler 14 ₂ is connected to the radio signal source T4.The division couplers 14 ₅, 14 ₆ are connected in the same way as in theexample in FIG. 6. The two other division couplers 14 ₇, 14 ₈ which mayalso be of the type described by reference to FIG. 2 (i=7, 8), aresimilarly installed so as to supply two signals to each of the receiversR2 and R4 from electric field components supplied by the duplexers 22_(H) and 20 _(V) and picked up by the horizontal dipole of the antenna12 and by the vertical dipole of the antenna 10 respectively.

The signals produced by the sources T1 and T3 are transmitted with LCPby the two antennas, and those from the sources T2 and T4 aretransmitted with RCP by the two antennas. It is thus possible to benefitfrom a gain in directivity for some at least of the sources. Forexample, if the distance d between the antennas and the differences inlength ΔL, ΔL′ are selected in the way previously indicated, an optimumgain in directivity will be obtained for the sources T1 and T2. It isalso possible to envisage some less than optimum choices which enablethe gains in directivity to be shared between the four sources. It isfurther possible to take advantage of the possibility of obtainingdifferent radiation patterns for the sources T1, T2, on the one hand andT3, T4 on the other hand, so as to create a multi-beam antenna systemwhich would transmit with a degree of isolation the signals from T1 andT2 into one portion of space and those from T3 and T4 into anotherportion, thus enabling an “electronic sectoring” of the coverage area.

The version example shown in FIG. 8 differs from that in FIG. 7 by theform of connection of the coaxial cables connected to the outputs D1 andD2 of the distribution couplers 14 ₁, 14 ₂: the coaxial cable connectedto the output D1 of the coupler 14 ₁ is moreover connected to the inputB4 of the polarisation coupler 14 ₄, whereas the output D2 of the otherdistribution coupler 14 ₂ is connected to the input A4 of thepolarisation coupler 14 ₄. In the case of FIG. 8, the four radio signalsources benefit from polarisation transmission diversity since thesignal from each of them is transmitted with LCP by one antenna and withRCP by the other antenna.

It is noted that the radio station of the invention, the operationalflexibility of which has already been stressed, has the additionaladvantage of being easily reconfigurable. Starting from a initialconfiguration such for example as the one on FIG. 1, the operator hasthe possibility of developing it as required by adding transceiver unitsto it, since the options for connection by coaxial cables enable a gainin directivity or in diversity to be obtained. Even in a configurationcomplete with four units TR1-TR4 such as the one in FIG. 7 or 8, theradio power from each of the four sources T1-T4 is fully radiated; thereis no undesirable dissipation in the impedance matching resistors.

The invention has been described above in the case where the antennas10, 12 are constituted by two crossed dipoles. The professional willappreciate that other antenna configurations could be used in thecontext of the present invention, so long as they enable two orthogonalelectric field components to be generated in response to two quadratureradio signals. A usable antenna could thus be constituted by a squareconductor pattern laid on a dielectric substrate, two adjacent sides ofwhich would receive the radio signals from the correspondingpolarisation coupler.

Moreover, it is possible to use hybrid couplers different from the oneillustrated in FIG. 2. In particular, there is no requirement for thedistribution couplers 14 ₁, 14 ₂ or division couplers 14 ₅-14 ₈ toproduce quadrature signals.

What is claimed is:
 1. A radio station, comprising: first and secondhybrid transmission polarisation couplers; first and second antennasrespectively associated with said first and second hybrid transmissionpolarisation couplers, each antenna being arranged to generate twoorthogonal electric field components in response to two respectivequadrature radio signals from the polarisation coupler associatedtherewith; at least one hybrid distribution coupler having a firstoutput connected to a first input of the first polarisation coupler anda second output connected to a first input of the second polarisationcoupler; and at least one radio signal source delivering a radio signalto a first input of the distribution coupler.
 2. A radio station inaccordance with claim 1, wherein said hybrid couplers are connected toeach other and to the first and second antennas in such a way that theradio signal delivered to said first input of the distribution coupleris transmitted by the first and second antennas in the form of tworespective radio waves circularly polarised in the same direction.
 3. Aradio station in accordance with claim 2, a distance between the firstand second antennas and lengths of coaxial cables connectingrespectively the first output of the distribution coupler to the firstinput of the first polarisation coupler and the second output of thedistribution coupler to the first input of the second polarisationcoupler are selected so as to obtain a gain in directivity for the radiosignal delivered to said first input of the distribution coupler.
 4. Aradio station in accordance with claim 3, wherein the distance betweenthe first and second antennas is about 0.92λ, where λ denotes the wavelength in the air of radio waves, and wherein a difference in lengthbetween the coaxial cable connecting the second output of thedistribution coupler to the first input of the second polarisationcoupler and the coaxial cable connecting the first output of thedistribution coupler to the first input of the first polarisationcoupler is of the form (n−ΔΦ/2π)λ″ where n is an integer, λ″ is the wavelength of the radio signals in the coaxial cables, and ΔΦ is a phasedifference between a portion of the radio signal delivered to said firstinput of the distribution coupler present at the second output of thedistribution coupler and another portion of the same radio signalpresent at the first output of the distribution coupler.
 5. A radiostation in accordance with claim 1, wherein said hybrid couplers areconnected to each other and to the first and second antennas in such away that the radio signal delivered to said first input of thedistribution coupler is transmitted by the first and second antennas inthe form of two respective radio waves circularly polarised in oppositedirections.
 6. A radio station in accordance with claim 1, furthercomprising another radio signal source delivering another radio signalto a second input of said distribution coupler.
 7. A radio station inaccordance with claim 1, comprising first and second distributioncouplers and first and second radio signal sources, the firstdistribution coupler having a first input receiving a first radio signalfrom the first radio signal source, a first output connected to a firstinput of the first polarisation coupler and a second output connected toa first input of the second polarisation coupler, and the seconddistribution coupler having a first input receiving a second radiosignal from the second radio signal source, a first output connected toa second input of the first polarisation coupler and a second outputconnected to a second input of the second polarisation coupler.
 8. Aradio station in accordance with claim 7, wherein said hybrid couplersare connected to each other and to the first and second antennas in sucha way that each of said first and second radio signals is transmitted bythe first and second antennas in the form of two respective radio wavescircularly polarised in the same direction.
 9. A radio station inaccordance with claim 8, wherein a distance between the first and secondantennas and lengths of coaxial cables connecting respectively the firstoutput of the first distribution coupler to the first input of the firstpolarisation coupler, the second output of the first distributioncoupler to the first input of the second polarisation coupler, the firstoutput of the second distribution coupler to the second input of thefirst polarisation coupler, and the second output of the seconddistribution coupler to the second input of the second polarisationcoupler are selected so as to obtain a gain in directivity for each ofsaid first and second radio signals.
 10. A radio station in accordancewith claim 9, wherein the distance between the first and second antennasis about 0.92λ, where λ denotes the wave length in the air of radiowaves, and wherein a difference in length between the coaxial cableconnecting the second output of the first distribution coupler to thefirst input of the second polarisation coupler (14 ₄) and the coaxialcable connecting the first output of the first distribution coupler tothe first input of the first polarisation coupler is of the form(n−ΔΦ/2π)λ″ where n is an integer, λ″ is the wave length of the radiosignals in the coaxial cables, and ΔΦ is a phase difference between aportion of the first radio signal present at the second output of thefirst distribution coupler and another portion of the first radio signalpresent at the first output of the first distribution coupler, andwherein a difference in length (ΔL′) between the coaxial cableconnecting the first output of the second distribution coupler to thesecond input of the first polarisation coupler and the coaxial cableconnecting the second output of the second distribution coupler to thesecond input of the second polarisation coupler is of the form(n′−ΔΦ=/2π)λΔ, where n′ is an integer, and ΔΦ′ is a phase differencebetween a portion of the second radio signal present at the first outputof the second distribution coupler and another portion of the secondradio signal present at the second output of the second distributioncoupler.
 11. A radio station in accordance with claim 7, wherein saidhybrid couplers are connected to each other and to the first and secondantennas in such a way that each of said first and second radio signalsis transmitted by first and second antennas in the form of tworespective radio waves circularly polarised in opposite directions. 12.A radio station in accordance with claim 7, wherein at least one of thefirst and second distribution couplers has a second input connected toanother radio signal source.
 13. A radio station in accordance withclaim 1, further comprising at least one receiver ensuring a diversitytreatment of two input radio signals, one of said input radio signalsbeing obtained from an electric field component picked up by one of thefirst and second antennas in accordance with a first direction, and theother input radio signal being obtained from an electric field componentpicked up by the other one of the first and second antennas inaccordance with a second direction orthogonal to said first direction.14. A radio station in accordance with claim 13, comprising at least tworeceivers each ensuring a diversity treatment of two respective inputradio signals, a first division coupler having an input receiving aradio signal obtained from an electric field component picked up by oneof the first and second antennas in accordance with the first directionand two outputs each delivering an input radio signal to a respectiveone of the two receivers, and a second hybrid division coupler having aninput receiving a radio signal obtained from an electric field componentpicked up by the other one of the first and second antennas inaccordance with the second direction and two outputs each delivering theother input radio signal to a respective one of the two receivers.